Amplifier



Oct 30, 1934. P. F. G. HOLsT Er AL AMPLIFIER Filed Jan. 28, 1953 l r 3500 wooo FREQUENCY lf( CYCLES PEF SEC.

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Patented Oct. 30, 1934 AMPLIFIER Poul F. G. Hoist, Riverton, and Rudolph A. Bierwirth, liaddcnfield, N. J., assignors to Radio Corporation of America, a corporation of Dela- Ware Application January 28, 1933, Serial No. 653,964

12 Claims. (Cl. 179-171) The present invention relates to amplifiers, and more particularly to audio frequency power ampliers of the type employing electric discharge devices of the so-called pentode type, operating into a load impedance, such as a dynamic loudspeaker device, which varies with frequency.

Electric discharge devices of the pentode type, for audio frequency power aniplier devices have five electrode elements including the usual cathl-ll ode, control grid, and anode, and in addition, a

screen grid electrode interposed between the anode and the remaining elements or electrodes, and a suppressor grid electrode between the anode and the screen grid electrode. The sup- 323 presser grid, located adjacent to the anode, is

:1J tween the screenY grid and the anode.

A device of the above type may be termed and is known as a constant current device, because in connection with a load circuit, variations in the load impedance normally have no appreciable l) effect upon the A. C. or signal current flow through it. In other words, Ithe device has'such a relatively high anode circuit impedance compared with the load impedance in that circuit that variations in the latter impedance are such 3.3 a relatively low percentage of the total circuit impedance that no appreciable change in the A. C. or signal current occurs, with variations in the load impedance.

The pentode type of power amplifier device has t the advantage in use, as is well known, of relatively high power output with a relatively low operating anode potential, together with a high amplincation factor. As ordinarily used, however, in a push pull amplifier stage, it has the disadvantage that parasitic oscillations frequently develop, a condition which greatly impairs the performance of the stage even if the oscillations are of an inaudible frequency. Heretofore, a customary method of suppressingsuch oscillations 53 has been to insert a resistor of relatively high value in the grid circuit of the push pull pentode stage to act as a suppressor means directly in series with the grids or control electrodes. This, however, has the further disadvantage that E5 whenever on signal peaks the grids tend to 'draw' appreciable grid current, added distortion is introduced because of the drop in signal potential introduced icy the suppressor resistor or resistors.

The pentode type of audio frequency power amplier device is characterized by the fact that it has a relatively high eiiiciency and a relatively high amplification factor, making it desirable as an output device for several applications, such for example, as in an output amplifier stage for driving a loudspeaker device in connection with radio receivers and the like. The usual loud speaker voice coil impedance is variable throughout the audio frequency range and its direct use as an output load for a push pull audio power amplifier stage employing electric discharge devices of the pentode type has heretofore been found to be unsatisfactory without the introduction of extensive corrective measures. The pen todo, furthermore, is substantially a constant current output device and such an amplifier tends to emphasize the higher audio frequency range,

In View of the advantages inherent in the use of electric discharge devices of the pentode type in audio power amplifiers, it is an object of the present invention to provide an improved push pull audio frequency power amplifier stage which is adapted to utilize electric discharge amplifier devices of the pentode type, or devices having a screen grid and a suppressor grid7 in addition to the usual three elements, without introducing the above named and other disadvantages heretofore experienced with such an amplifier stage.

It is a further object of the present invention to provide an electric discharge audio frequency amplier stage, adapted to employ/,electric discharge devices of the pentode type, which is further adapted to operate in conjunction with a load impedance which is variable throughout the audio frequency range. ,i

It is a still further object of the present invention to provide a push pull audio frequency power amplifier adapted to einploy electricsdischarge devices of the pentode type, which may operate as a single audio frequency amplifier stage between an electric discharge power detector and a frequency variable impedance device such as a loudspeaker or other sound producing device of like nature.

In accordance with the invention, a pair of electric discharge devices of the pentode type, each having a screen grid and a suppressor grid, are arranged in push pull relation as an output stage. between a driver stage including an electric discharge device, and a variable load device such as a loudspeaker. The output stage is provided with a push pull input circuit including a coupling transformer and a push pull output circuit including a second coupling transformer. The associated and coupled circuits for the output and driver stages are arranged to have certain impedance values, and means are employed in conjunction with the input and output circuits of the push pull output stage to produce a suitable over-all operating characteristic for the combined amplifier and sound produced, thereby suppressing any tendency to produce oscillations in the amplifier and to prevent high frequency signal distortion because of the constant current output characteristic of the pentode and the increasing high frequency impedance oi the loudspeaker voice coil.

The invention will, however, be better understood from the following description when taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a schematic circuit diagram of an amplifier embodying the invention;

Fig. 2 is a curve diagram showing an operating characteristic of the output device employed 1n Fig. 1; and

Figs. 3 and ll are similar curve diagrams showing different operating characteristics of the amplier of Fig. 1.

Referring to Fig. 1, a pair of electric discharge devices or tubes 5-5 are arranged in push pull relation to each other between a push pull input circuit 6-6 and a push pull output circuit 7-7. The input circuit includes an interstage coupling transformer' 8 and the output circuit 9 of a driver stage indicated by an electric discharge device or tube 10. The output circuit includes an output transformer l1 and a source of load represented by a loudspeaker voice coil 12.

It will be noted that the push pull amplifier devices 5 are each provided with av cathode 13, having a common heater circuit represented by leads 14, a control grid 15 connected with the input circuit, and a screen grid 16 connected with the high potential side of a suitable source of operating potential indicated by the supply leads 18, across which is connected the usual bleeder or potential supply resistor 17.

A suppressor grid 19 in each amplifier device is interposed between the screen grid 16 and the anode 20. The suppressor grids in the present example are connected directly to the cathodes 13 as indicated, while the anodes are connected with the output circuit 7. The anodes receive operating potential from the leads 18 through the primary winding 21 of the output transformer 11 which is center tapped for push pull operation. The secondary 22 of the transformer is connected with the loudspeaker 12, all as indicated.

In the input circuit, the interstage or coupling transformer 8 is provided with a primary winding 23 which is included in the output or anode circuit 9 of the device 10. while the secondary indicated at 2d is center tapped and connected to the cathodes through a bias lead 25 and a suitable self bias resistor 26 having a oy-pass capacitor 2'?. The cat-nodes are, in. turn, connected to the negative side of the potential supply leads 18 through the low potential return lead indicated at 28.

Referring now to Fig. 2, the usual impedance characteristic of an electrodynamic loudspeaker such as the loudspeaker 12 is indicated by the curve 29 drawn with respect to frequency in cycles per second and impedance over the audio frequency range indicated. It will be seen from the curve 29 that the impedance of the usual loudspeaker voice coil as employed in connection with power amplifier stages such as described thus far in connection with Fig. 1, is highly variable, and when employed in connection with pentode type output tubes this characteristic results in a sound output which is not in accordance with the signals applied to the input terminals of the amplifier.

An amplifier device of the pentode type has an internal anode impedance which is relatively high compared to the optimum external load impedance. Therefore, as hereinbefore pointed out, it is obvious that the device is essentially a constant current device and that the voltage characteristics on the load will be dependent on its impedance characteristic.

If the output transformer 11 were an ideal transformer, the primary winding 2l would reflect to the anode circuit a load impedance which would be equal to the loudspeaker impedance as shown in Fig. 2 multiplied by a constant factor, i. e., the square of the turns ratio. Consequently, the voltage response characteristic of the load would have essentially the same shape as the impedance characteristic 29 of the loudspeaker, which is not desirable from the standpoint of tone quality.

In view of the foregoing, it is evident that correction means must be applied to the coupling system to compensate for variations in the load impedance of the load device.

As an initial step in correcting for the variations in impedance of the load, a resistance filter is connected across the output circuit, preferably in the primary side of the output transformer 1l as indicated at E30-31. This filter serves to attenuate the signals transmitted by the amplier in the higher audio frequency range, thereby to compensate for the increased response characteristic which will be found across the load device or loudspeaker in the higher frequency range as indicated by the curve 29 in Fig. 2. The result obtained by means of this loading is shown by the curve 32 in Fig. 3.

It has been found, however, that a filter impedance across the load circuit is not sufficient to completely compensate for the response characteristic of the loudspeaker or load device when operating in conjunction with the pentode type ampliiier devices. It will be seen that in the lower frequency range a decided peak in the response characteristic is present, which may result i a high degree of distortion, and in conjunction with an ordinary dynamic loudspeaker, for example, the response of the moving element may be such as to result in physical damage to it on higher signal peaks.

It has been found, however, that if the prig mary winding 21 of the output transformer 11 is reduced in impedance to a suitable low value as compared with the reflected impedance of the load device 12, the response characteristic of the amplier may be smoothed to obtain a charac- 1 lid devices, and an output coupling transformer having a relatively low primary impedance with respect to the reected load impedance, the response characteristic of the output devices in conjunction with the usual load device or loudspeaker voice coil may be compensated for to provide a smooth, distortionless audio frequency sound output.

The over-all audio frequency sound output characteristic of an amplifier and sound production device, or loudspeaker, employing push pull ampliei devices of the pentode type, is dependent, however, not only upon the design of the output circuit and the elements therein, but upon the design of the input circuit and the elements therein.

In the present example the amplifier device 10, which may be a high power detector, is coupled to the push pull output device 5 through the interstage coupling transformer 8 which may have a step-up ratio of 1 to 2 over all. Connected across each half of the secondary of the transformer is a condenser 34, and across the entire input circuit is connected a second resistance filter means including a condenser` 35 and a variable resistor 36 connected in series with each other.

The condensers 34 are for two purposes. First, to prevent parasitic oscillations from being set up in the amplifier, since in series they form a shunt high frequency attenuating means across the input circuit, and secondly, to obtain a desired high frequency response characteristic from the amplifier and output devices. In conjunction with the condensers 34, the lter means 35--36 serves two purposes-primarily as a tone control means wherein the condenser 35 may gradually be placed in shunt relation to the input circuit as the re sistor 36 is reduced in value. The secondary function of the filter means 35-36 will be hereinafter described.

The interstage coupling transformer 8 in the input circuit is provided with a relatively high primary impedance and a relatively high leakage reactance, as indicated in the drawing. With a primary winding of this character and a high leakage impedance, it has been found that a relatively small transformer may be designed, while having a desired audio frequency response characteristic.

The leakage reactance resonates With the condensers 34 at a signal frequency above which it is desired to cut off cr attenuate the audio frequency response characteristic of the ampliiier. In the present example the primary Winding 23 may have an inductance of approximately 115 henries, and with a 1 to 2 step-up ratio the condensers 34 may have a capacity of substantially 400 micro-microfarads. With the constants as above indicated, the input circuit is resonated at approximately 3500 cycles, resulting in a signal out oli at a slightly higher frequency. The system then operates as a low pass filter.

As compared with known methods for reducing parasitic oscillations, including, as hereinbeiore mentioned, the use of resistors in the input grid circuits of an amplifier, the use of the low pass lilter system above described has the advantage that no added distortionis introduced, while the parasitic oscillations are eiiectively attenuated.

The high leakage reactance of the interstage transformer 8, furthermore, permits a satisfactory operation of the tone control filter 35-36 for further increasing the low pass lter effect. .It will be seen that `as the resistor 36 is reduced in value substantially to Zero, the cut oif point or the amplifier` is moved to a low frequency Value. The eect of the low pass iilter in connection with the input circuit is shown in Fig. 4 by the curves 37 and 38, in which curve 37 represents the operating characteristic of the amplifier when the tone control 'resistor 36 is at maximum value, showing the cut oi characteristic of the ampliiier with the resonating condensers 34 alone, while the curve 38 shows the reduced response characteristic when the tone control resistor 36 is reduced to minimum value to cause the condenser 35 to have full eifect in the filter in adidtion to the ccndensers 34, while it serves also as the tone control means.

To provide the cut off characteristic indicated inthe curves of Fig. 4 the condenser 35 may have a value of substantially 3,090 micro-microfarads, and the resistor 3d may have a value of 200,000 ohms maximum in the circuit shown and described.

From the foregoing description it will be seen that in accordance with the invention an ampli fier is provided, which may compensate for the operating characteristics of pentode type ampli iier devices and of the usual sound producing devices employed in the output circuit thereof, whereby undesirable frequency response characteristics tending to cause distortion in the final sound output may be removed or compensated.

The amplifier includes an input transformer if having a high primary impedance and a high leakage reactance tuned to provide a low pass iilter in the input circuit, combined with tone control means for varying the cut off point of the lter. Parasitic oscillations tending to be built up in the amplier because of the high impedance of the pentode amplifier devices are prevented without resorting to the use of series resistors which tend to introduce distortion on peak signals.

The attenuation of the higher frequency portion Vof the audio frequency range in the input circuit is supplemented by a ilter means in the output circuit which compensates for the high frequency response of the amplifier devices.

Further attenuation is introduced by the low im- Q pedance of the primary winding of the output transformer and the desired over-all response is thus substantially in accordance with the signals applied to the driver stage.

In conjunction with an ampliiier as shown and described, a loudspeaker will neither be subjected to distortion in the lower portions of the audio frequency range nor will it be caused to generate distorted signals in the higher audio frequency range, '1n each of which portions of the audio frequency range, in conjunction with a pentode type of amplifier, it is normally most responsive.

We claim as our invention:

l. In, an vaudio frequency power ampliiier, a 'Y pair of balanced electric `discharge amplifier devices of the high impedance constant current type, audio frequency coupling means providing a low pass lter input circuit for said devices, a frequency variable load impedance and a coupling transformer between said devices and said load impedance having a lower primary impedance than the reflected load impedance.

2. In an audio frequency power ampliiier, an

input circuit, tuned audio frequency circuit coupling means providing a low pass filter in said circuit, tone control means connected with said circuit to vary the cut orf point of said filter, a

pair of electric discharge ampliiier devices having control grids connected'with said input circuit and having suppressor and screen grids, an output circuit for said devices, and an output tranfcrmer having a primary winding in said output circuit, the impedance or" said winding being low y with respect to the reflected load impedance for the amplifier.

3. In an audio frequency amplifier, a pair of balanced electric discharge devices of the type having a screen grid and a suppressor grid, of a balanced input circuit therefor' including a coupling transformer having a relatively high leakage reactance, means connected across the secondary for tuning the leakage reactance, thereby to provide a low pass filter in the input circuit, a balanced output circuit including an output transformer a load impedance, said transformer having in said output circuit a primary winding of relatively low impedance with respect to the reflected load impedance in circuit therewith.

4. In an audio frequency amplifier, a pair of balanced electric discharge devices of the type having a screen grid and a suppressor grid, of a balanced input circuit therefor' including a coupiing transformer having a relatively high leakage reactance, nieans connected across the sccondary for tuning the leakage reactance, thereby to provide a lcw pass filter in the input circuit, means for varying the cut off frequency of said lcv.7 pass filter, a balanced output circuit including an output transformer having a primary winding of relatively low impedance with respect to the reflected load impedance for the amplier.

5. in ampliner including in combination, a push pull output stage and a driver stage therefor, a load circuit for the output stage,4 a coupling transformer vterposed between the output stage and the load circuit having a relatively low primary impedance with respect to the reflected load impedance for the ainpliner, a second coupling transformer interposed between the driver stage and the output stage, said transformer having a relativelyV high primary impedance with respect to the output impedance of the driver stage and having a relatively high leakage reactance, and means connected with the input circuit of the output stage and said last named transformer for tuning said leakage reactance to provide a low pass filter in the input circuit of the output stageA 6. In an audio frequency power amplifier, a push pull input circuit including an. input transformer having a relatively high leakage reactance and means for tuning said leakage reactance to a low cut off frequency, thereby to provide a low pass filter, a tone control filter means associated with said low pass filter for varying the cut off frequency of said last named filter means, and an output anode circuit including an output transformer having a primary winding the impedance of which is relatively low with respect to the load impedance for the amplifier'.

7. ln an audio frequency amplifier, the combination with a pair of electric discharge devices each having a screen grid and a suppressor grid, and means providing a load circuit for the amplifier having a variable impedance characteristic with respect to frequency, of coupling means for said amplifier devices adapted to compensate cerpcrtions of the response characteristic of said devices and load circuit, whereby a substantially uniform over-all frequency response characteristie is obtained, said means including a balanced input transformer for said amplifier devices hava relatively high leakage reactance tuned to a low cut off frequency, whereby a low pass filter provided in the amplier input circuit and said transformer' having a relatively high primary impedance, a balanced output circuit for said devices, and an output transformer for said amplifier devices having a secondary winding connected with the load circuit and in said output circuit a balanced primary winding of relatively low impedance with respect to the load impedance reflected over 'into said output circuit.

8. in an audio frequency power amplier, ay

ance than the reflected load impedance in said i.

output circuit, at the low frequency end of the audio frequency range.

9. The combination with an electric sound producing device having a variable load characteristie in response to audio frequency signals, of

an audio frequency amplifier adapted to operate with a pair of electric discharge devices of the pentode type in the output stage of said amplifier to supply amplied audio frequency signals to said sound producing device, said output stage including a balanced input circuit, an input coupling transformer for said circuit having a high primary impedance and a high leakage reactance, means for tuning said leakage reactance to provide a predetermined cut-off characteristic inthe high audio frequency range of said amplier, a balanced output circuit for said stage including an output transformer having a balanced primary Winding and a secondary Winding, and a load circuit connected with said secondary winding and including said sound producing device, said primary Winding having a lower impedance within a predetermined low audio frequency range than the load reflected over from said secondary into said output circuit.

10. The combination with a loud speaker, of a driver amplier therefor adapted for pentode or constant alternating current operation without appreciable audio frequency peaks in the loud speaker response characteristic, said amplifier comprising an input transformer having a high primary impedance and a high leakage reactance tuned to cut-off the high frequency peak of the loud speaker response characteristic, and an output transformer having a primary impedance lower than the load of the loud speaker refiected into circuit therewith, thereby to cut-oir the low frequency peak of the loud speaker response characteristic.

11, The combination with a loud speaker, of -1 a driver amplifier therefor adapted for pentode or constant alternating current operation without appreciable audio frequency peaks in the loud speaker response characteristic, said arnpliier comprising an input transformer having a high primary impedance and a high leakage reactance, means for capacity loading the secondary of said transformer to tune said leakage reactance thereby to cut-off the high frequency peak of the loud speaker characteristic, an out- Y f4 put circuit, an output transformer in said circuit having a primary impedance lower than the load of the loud speaker reflected into circuit therewith, thereby to cut-off the low frequency peak of the loud speaker characteristic, and -a resistance filter connected across said output circuit for further atenuating the high frequency response characteristic of the loud speaker.

l2. The combination with an electric sound peak, and a balanced output circuit for said devices including an output transformer having a relatively 10W primary impedance with respect to the reflected load of the sound producing den vice in circuit therewith Within an audio frequency range including said low frequency peak, thereby to cut-off said low frequency peak.

POUL F. G. HOLST. RUDOLPH A. BIERWIRTI-I. 

